Wax mixtures or paraffin mixtures which can be used, for example, in the textile and packaging industries, in cosmetics, food technology or pharmaceuticals, frequently contain microcrystalline paraffins. Microcrystalline paraffins or waxes consist of mixtures of hydrocarbons which are solid at room temperature. Saturated hydrocarbons are concerned here, with a carbon chain length of about 25–100. Microcrystalline waxes, which contain isomeric paraffins as essential compounds, that is, branched paraffins (isoparaffins, i-paraffins) with a higher number of carbon atoms, are at present recovered from selected, high-molecular petroleum products (Ullmann's Encyclopedia of Industrial Chemistry, VCH Publishing Co., Weinheim 1996, Vol. A 28, 18–145). Microcrystalline paraffin recovered from petroleum contains, besides branched iso-alkanes, n-alkanes, naphthenes, and also aromatics.
Paraffins which according to the Fischer-Tropsch process are produced at high temperature from pure CO and H2 synthesis gas in the presence of a catalyst, have on the contrary substantially normal-chain alkanes and only small proportions of branched alkanes, and are free from naphthenes, aromatics, and also from sulfur and oxygen compounds.
Theoretically, such Fischer-Tropsch paraffins, of the range of about 30–100 carbon atoms, are to be converted by isomerization into microcrystalline waxes. The main problem of the conversion of such long-chain paraffins into i-paraffins is the competing hydrocracking reaction. Since the known catalysts work at relatively high reaction temperatures, a large part of the paraffins is not only isomerized but also subsequently split to smaller fragments, which have to be separated as byproducts (A. B. R Weber: Hydroisomerization of paraffin wax, thesis, Excelsior Publishing Co., S'Gravenhage 1957). Corresponding proposed methods are therefore based on converting the Fischer-Tropsch product catalytically, with partial isomerization and partial cleavage, at temperatures at which liquid products arise, which are suitable for use as lubricating oils with high viscosity index.
From WO 01/74971, it is known to carefully isomerize a Fischer-Tropsch product with wide boiling range and also containing liquid products, and to recover a wax from the liquid hydroisomerizate by distilling off the lighter fractions. The hydrogenating temperature is given with a spread of 204–343° C. (however, example: 348° C.); the lower temperature range, not supported at all by examples, appears to be questionable as regards practicability. The high-boiling fractions of the raw material are blended with the wax recovered in this way. The catalyst is typically given as a cobalt-molybdenum catalyst on aluminosilicate. Zeolite Y or ultra-stable zeolite Y are given as suitable zeolites. It appears to be disadvantageous in this proposal that the additional process step of distillation has to be used, making the production of the soft microcrystalline wax more expensive.
The present methods of production of microcrystalline paraffins are not yet technically satisfactory. A catalyst is therefore desirable which can selectively convert the solid Fischer-Tropsch paraffin to microcrystalline paraffins in a single process step.